Various types of weaving looms which operate on a single-system basis are so constructed that a shed is opened, extending over the entire width of the fabric to be made. A movable back rail can be used in order to compensate for variations in warp thread tension as the shed is formed. These variations in tension are caused by the heddles deflecting the ward threads - or a portion thereof--from the plane of the fabric towards one side or the other. Thus, the length of the path of the thread from the back rail to the wind-up fabric roll changes.
It has been proposed--see German Patent Disclosure Document DE-OS No. 33 46 030--to decrease the tension applied to the warp thread by providing a thread deflection element which contacts the warp threads, and which is supported on at least one elastic element, for example a cushion or pillow filled with a compression medium, or on a spring of the back rail, to permit some resilient deflection. The thread deflection element can be formed of a single beam or rod or bar extending over the width of the fabric to be made, and pivotable about an axis parallel to the width of the fabric. Alternatively, it has been proposed to subdivide the beam or rod into a plurality of similar narrower units made of stiff individual strip or rod elements, located essentially without gaps or spaces in a row adjacent each other, each one supported on an elastic element. The elastic element is formed by a straight circular cylindrical pipe or tube, open at the ends, of elastic material, such as soft rubber, made with thick walls, so that its stiffness for elastic support of the individual support or deflection elements is of suitable strength. It is difficult to prevent penetration of the tensioned threads, which are laterally not guided, into the stiff individual strip units. Such a back rail or back rest is suitable, apparently, only for weaving looms in which the shed is opened in advance of each introduction of the shuttle or weft thread over the entire width of the fabric. This is standard in weaving machines of a single-system arrangement, and such tensioning equalization apparatus is only suitable therefor.
Multi-system of multi-feed or unit weaving looms have a plurality of shuttles which are introduced, longitudinally staggered, between a shed which moves along the width of the fabric. The multiple shuttles simultaneously introduce the weft threads. The shed has the form of a progressive wave. If a back rail or rest is used in such a weaving machine which is resiliently supported, while extending over the width of the fabric, as proposed, for example, in German Pat. Nos. 1,066,958 and 1,287,526, effective equalization of the warp threads cannot be obtained thereby. Consequently, a properly and precisely controlled engagement of the weft thread is no longer insured since the corresponding edge of the fabric at the initial point of the weft thread within the closed shed moves and, consequently, the engagement pressure of the weft thread tends to be reduced, that is, the weft thread yields. A predetermined minimum engagement pressure of the weft thread is required at the initial position of the weft thread upon its entry into the shed.